The present invention relates to continuous ink jet printers and more particularly to improved constructions for stimulating synchronous drop break-up of the ink jets issuing from elongated arrays of orifices in such printers.
In continuous ink jet printing, ink is supplied under pressure to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s). The ink is expelled from the orifices in jets which break up due to surface tension in the ink into droplet streams. Ink jet printing is accomplished with these droplet streams by selectively charging and deflecting some droplets from their normal trajectories. The deflected or undeflected droplets are caught and re-circulated and the others are allowed to impinge on a printing surface.
To selectively charge the ink droplets, it is desirable to stimulate the ink jets to accurately control the locations that the droplets separate from the ink jets downstream from the orifice plate. Such stimulation is provided by applying a vibration to the ink, for example, by vibrating the orifice plate. Stimulation also maintains uniform drop size and drop spacing as well as controlling the location of drop separation. It is also desirable that the droplets in all of the jets separate at the same time from their respective jets, this is called synchronous stimulation. Such synchronous stimulation simplifies the problem of drop charging, since each drop in each jet separates from the jet at a precisely predictable time period allowing accurate drop charging and placement and avoiding printing errors due to improper droplet charging.
Synchronous stimulation of an array of ink jets at high frequency (e.g., above 40 kHz) is difficult when the array length is greater than xc2xd xcex, where xcex is the wavelength of an acoustic wave at the stimulation frequency f0. For a bulk acoustic wave, the wavelength is given by xcex=CB/f, where CB is the bulk velocity of sound given by CB=B/P where B is the bulk modulus of the material and P is the density of the material. For stainless steel, which is a currently favored material for synchronously stimulated continuous ink jet print heads, CB is about 5,000-6,000 m/sec, resulting in xcex≈11 cm at 50 kHz. Thus, for ink jet arrays longer than about 5 cm, it is difficult to achieve synchronous stimulation. This is so because the print head can vibrate in many different modes which are a function of its size.
As the physical dimensions of a print head increase, the number of vibrational modes increases, the relative frequency difference between the vibrational modes decreases and the modes become crowded in the operational frequency range of the print head. When there are vibrational modes in the print head that have a frequency close to the desired stimulation frequency, a phenomenon called mode coupling occurs and energy delivered to the print head to vibrate the print head in a desired mode to stimulate the ink jets causes the print head to be excited in other undesirable modes, thereby dispersing the stimulating energy, and disrupting the synchronous stimulation of the jets. For example, to operate a print head having an array of orifices xc2xd cm in length at 50 kHz the print head may be shaped such that its length perpendicular to the array is xc2xd xcex (about 5 cm) and its other dimensions are as small as possible. With this shape (long in the direction parallel to the ink jets) the print head has very few other vibrational modes near 50 kHz and hence mode coupling does not occur. A print head of this type is shown in U.S. Pat. No. 4,683,477 issued Jul. 28, 1987 to Braun, et al.
When the orifice array is made larger, for example, 10 cm, the print head has many other modes near 50 kHz which must be suppressed for proper operation of the print head. U.S. Pat. No. 4,999,647 issued Mar. 12, 1991 to Wood, et al, discloses an ink jet print head having a series of slots through the print head body to divide the body into a plurality of approximately identical dilatational regions. These slots have the effect of decreasing the mode coupling between the desired vibrational mode necessary for synchronous stimulation and undesired modes that decrease efficiency and frustrate synchronous stimulation. As printing speeds are increased, it becomes desirable to stimulate the ink jets at increasingly higher frequencies. It has been found, however, that print heads of the type shown in the ""647 patent cannot be synchronously stimulated much above 100 kHz before mode coupling again becomes a serious problem. At such high frequencies, the problem of mode coupling is compounded by the driving action of the drive crystals. When excited by the electric field, the piezoelectric transducers modulate in both the length and width directions. Hence, the piezoelectric transducers can excite vibration in not only the desired direction but also in the perpendicular direction. As a result, they couple into undesirable vibration mode.
A need has therefore been identified for a print head for a continuous ink jet printer which can be synchronously stimulated above 100 kHz.
It is the object of the present invention to provide a print head for a continuous ink jet printer that can be synchronously stimulated at frequencies above 100 kHz. It is another object to provide a print head that exhibits reduced mode coupling during stimulation.
The objects are achieved according to the present invention by stimulating an orifice plate defining an elongated array of orifices in a continuous ink jet print head with a shear mode piezoelectric transducer. Since a piezoelectric transducer does not exhibit substantial vibrational mode coupling when driven in a shear mode, the problems noted above with respect to the prior art are solved. For example, a piezoelectric ceramic crystal cut to a length of 7.5 cm with a 0.6 cmxc3x970.48 cm cross-section driven in a shear mode has a resonance near 200 kHz with its second harmonic at or near 400 kHz without any other resonances in that range. In contrast, when operated in its thickness mode, the same crystal will couple into vibrations at a very large number of dilatational and bending modes in the range of frequencies between 200 and 400 kHz. The advantage achieved by the present invention is the ability to operate a long (greater than several centimeters) ink jet print head at frequencies greater than 100 kHz.
Other objects and advantages of the invention will be apparent from the following description and the appended claims.